This invention relates to a superconducting magnet having a superconducting coil wound on the inner circumference of a coil case, and more particularly to a superconducting magnet having an auxiliary shield for shielding the superconducting coil against radiant heat from the exterior.
A conventional superconducting magnet of this type is described in an article entitled "A Thin Superconducting Magnet Solenoid wound with the Internal Winding Method for Colliding Beam Experiments", published in "Journal de Physique", January, 1984 Supplement, PP. Cl-337 to Cl-340, the contents of the article being originally presented at the 8th International Conference on Magnet Technology held at Grenoble, France, September 5th-9th, 1983.
A partial drawing of the conventional magnet apparatus is illustrated in FIG. 1. Referring to the drawing, a coil case 1 which is made of aluminium is cooled by a cryogenic refrigerant such as liquid helium flowing through holes formed therein or through tubing provided therearound. On the inner circumference of the coil case 1, there is provided a cylindrical superconducting coil which is wound by the coil lead 2a. An electrically insulating layer 3 is disposed between the inner surface of the coil case 1 and the outer circumference of the superconducting coil 2. This layer is made of epoxy resin or glass wool and is bonded to both the coil case 1 and the coil 2 so as to achieve good heat conductivity therebetween.
A main heat shield 4 surrounds the coil case 1 and the superconducting coil 2 with a vacuum 5 maintained therebetween. The main heat shield 4 is cooled by liquid nitrogen flowing through tubing 11 disposed on the surface thereof.
The above members are disposed inside a vacuum container 6 in which a vacuum 5 is maintained. The main heat shield 4 is supported by tensile support members 10 which are secured to the inner walls 6a of the vacuum container and which have a high thermal resistance. The coil case 1 is similarly supported inside the main heat shield 4.
The operation of the illustrated magnet is as follows. The coil case 1 is cooled by the liquid helium and then cools the superconducting coil 2 wound on the inner circumference thereof to about 5K (Kelvin's temperature) and maintains the cryogenic state. A vacuum 5 is maintained between the superconducting coil 2 and the main heat shield 4, and between the main heat shielding 4 and the vacuum vessel 6 so as to provide heat insulation. When a current flows through the superconducting coil 2, a magnetic field is generated. This magnetic field provides a radially outward force on the superconducting coil 2. To support this electromagnetic force, the coil case 1 is disposed on the outer circumference of the superconducting coil 2. As there is no mechanical supporting member on the inner surface of the superconducting coil 2, the weight of the magnet can be decreased and the permeability of particles can be increased in an experiment of high-energy physics.
The conventional apparatus having the above-described structure has the disadvantage that when the superconducting coil 2 becomes unbonded from the coil case 1 due to repeated stresses experienced over a long period of use, a large thermal resistance develops between the coil case 1 and the superconducting coil 2 and the superconducting coil 2 is irradiated by radiant heat from the main heat shield 4. The temperature of the coil 2 therefore rises, and a predetermined performance can not be obtained.